This invention relates to a method for axially moving a tube in a borehole in the ground according to the introductory portion of claim 1. The invention further relates to a device for axially moving a tube into a borehole in the ground according to the introductory portion of claim 19.
GB 591 922 and GB 596 715 disclose a method for axially moving a tube carrying a drill in a borehole in the ground, wherein the tube is moved simultaneously along and about its axis in a series of alternating, angularly opposite, rotating movements within a limited angular range of rotation of less than 180xc2x0.
Methods and devices according to the preamble of claims 1 and 19 are known from practice, for instance for axially inserting a tube in a borehole or for axially retracting tube from a borehole. Such a tube may be used for the drilling process and simultaneously for the protection of the newly drilled wellbore. This technique is commonly referred to in the field as casing drilling. Such a borehole can be used for extracting oil or gas and for other purposes, such as for the extraction of salt or geothermal energy and also for civil engineering purposes such as laying pipelines under rivers.
When using the known methods and devices, usually a first tube part is inserted into a borehole in the ground provided by a drill. Subsequently further tune parts are coupled to the upper end of the tube reaching into the borehole, via a screw and/or clamp coupling. As the tube is introduced further into the ground, successive tube parts, which can each be composed of one or more pre-connected tube joints, are connected to the proximal end of the composed section of tube end projecting from the ground until the tube has reached its final length. When the tube is removed, this method is essentially reversed. the frictional force between the tube and its surroundings with a tangential component, thus reducing the axial frictional component, which substantially decreased the axial force needed to insert or retract the composed tube into or from the borehole. It shall be clear that also a single length of tube can be used instead of connected tube parts.
A disadvantage of the known method and device for axially moving a tube in this manner is that, due to the continuous rotation of the tube, it is a problem to provide solid-state, safe connections between the stationary ground and apparatuses rotating jointly or solidarity with the tube. In particular, during drilling operations a swivel connecting the tube to a mud supply is to be provided with an elaborate rotational seal. The same holds true for e.g. hydraulic connections to a drill carried on the tube or pneumatic connections to a packer connected to the tube. In particular, providing a spark-free electrical connection between devices rotating substantially jointly with the tube and the ground in the often explosion endangered environment near a borehole has proven to be a problem.
In practice, due to the problem of providing a reliable connection, often the rotation is stopped, when e.g. a drill needs to be retracted or when an inspection apparatus has to be lowered. This greatly increases the risk of the tube getting stuck due to the settling of the ground or due to a phenomenon known in the field as differential sticking.
In the currently known art of casing drilling, the direction of the tube is controlled by devices protruding from the lower end of the tube. The application of such devices is costly and the measure of control is limited. As such it is currently not feasible to apply casing drilling for high angle or near horizontal holes.
It in an object of the invention to circumvent, at least to a considerable extent, the drawbacks associated with the above method and device.
This object is achieved according to the present invention by carrying out a method for axially moving a tube in a borehole in the lithosphere in accordance with claim 1. The invention further provides a device according to claim 19 which is specifically adapted for carrying out such a method.
By twisting the tube about its axis in a series of alternating, angularly opposite, rotating movements within a limited angular range of rotation, it is possible to use relatively simple, flexible connections extending between fixed couplings to provide a connection between devices rotating jointly or solidarily with the composed tube and the stationary ground. An example of such a flexible connection extending between fixed couplings is e.g. a length of flexible hose or pipe fixed with simple clamps on each end for fluid connections or a length of insulated cable provided with plugs on each end for co-operation with sockets to provide an electrical connection.
The limited rotational angle of this xe2x80x9coscillatingxe2x80x9d rotational movement will prevent the flexible connections from becoming wound-up too far as would be the case with an increasing rotational angle caused by the rotational movement of the prior art.
Furthermore, during insertion or retraction of a device, e.g. a drill, a packer, a sensor or an inspection apparatus, through the inside of the tube, the tube can be oscillated continuously, thus greatly reducing the risk of the tube getting stuck.
The angular range of rotation can be preselected to comprise approximately one full rotation of 360, e.g ranging from xe2x88x92180xc2x0 to +180, or 0xc2x0 to 360xc2x0. The range can also comprise more than one full rotation, i.e. include an angle of more than 360xc2x0 in e.g. the interval from 0xc2x0 to 720xc2x0 or xe2x88x92360xc2x0 to +360xc2x0 or a multiple of this angle or less than one full rotation, e.g. 0xc2x0 to 180xc2x0 or xe2x88x9290xc2x0 to +90xc2x0. Preferably, less than 5 full rotations in one direction, e.g. left hand rotations, are followed by the same number of full rotations in the opposite direction, e.g. right hand rotations.
The oscillating movement can be such that each alternating angular movement is substantially equal in magnitude, such as to achieve a symmetric pattern. However, it is also possible to perform a series of alternating rotating movements that are not equal in magnitude, e.g. a series of alternating movements that provide a gradual angular drift which may have an oscillating pattern so that the total angle of rotation remains within a limited angular range at all times or be a drift in one sense so that the total angle of rotation remains within a limited angular range for a limited duration only. Such a duration may for instance be of sufficient length to perform operation on or with the tube, such as steering the drilling direction or connecting an end of tube material to the tube.
It shall be clear that within this context the rotational angle within the pre-selected angular range is defined as an absolute angle of rotation of the tube about its axis, relative to the ground.
Furthermore, it shall be clear that when axially moving the tube, a first series of alternating angularly opposite rotating movements performed within a first pre-selected angular range of rotation can be followed by a second series of such movements within a second range.
The frequency of the opposite movements or oscillations is preferably less than several oscillations per minute, typically less than 10 oscillations per minute and/or can be chosen to match the natural frequency of the tube in the ground. In the proposed method the oscillations are preferably performed at 0,1 or 0,05 Hz. The method however does not exclude the application of a higher frequency of oscillation, e.g. in a typical range from 1 to 50 HZ.
In another advantageous embodiment of the invention a welding or cutting apparatus oscillates substantially jointly or in unison with a composed tube and a rotating tube part to be connected therewith, respectively to be cut therefrom. This enables continuous insertion or retraction of the tube into/from the borehole, as will be elucidated further. It should be noted that welded or other rotationally rigid connections between the successive tube parts are especially advantageous in view of their increased resistance to become undone by the angularly opposite rotational movements cared to the screw connections generally used to couple successive casing or tubing parts. The rotationally rigid connections between the tube elements allow precise control of the relative orientation of the tube end below the surface and the tube part visible at the surface. The use of an axial line provided on the outside of the tube, e.g. by inscription, and precise measurement of the angle between the line on each consecutive tube section will provide an exact knowledge of the orientation of tube end below the surface.
By measuring the torque exerted on the tube at the surface while performing angularly symmetric oscillations, a mid-point position can be determined, characterised by the mid-point of the lower torque values. The azimuth of this mid-point position relative to the azimuth of the inscribed line is indicative for the azimuth of the tube end. The tube end azimuth determined in this way can be corrected for the reactive torque of the drilling tool suspended in the tube end, if necessary.
Another important advantage of the consecutive angularly opposite movement is that, by performing the consecutive movements within a circle segment, a tube having a bent tip can be steered while inserting it axially into the ground. Instead of a bent tip, it is also possible to use other kinds of devices standing-off asymmetrically from the axis A at the end of the tube tip. In order to change the direction of the borehole to be drilled, i.e. the azimuth and angle of the borehole, the oscillating range is chosen to be symmetrical about a predetermined tube end azimuth and to include less than 360xc2x0 rotation of the tube end, for instance a range from xe2x88x9245xc2x0 to +45xc2x0. These small oscillations achieve that the cutting process at the tube end preferentially removes rock or ground in a circle segment corresponding to the desired hole direction This process is continued until the desired tube end hole angle in the desired tube end azimuth is achieved. In order to continue the borehole at the same hole angle in the desired tube end azimuth, the oscillations and rotation are adjusted such that the cutting process at the tube end removes equal amounts of rock in all directions. The borehole can thus be steered in the desired direction using surface measurements based on the azimuth of the line and inclination measurements at the tube end. Preferably, a drill is used of which the rate of material removal is independent of the direction or speed of rotation, e.g. a hydraulically-powered chisel.
In addition, by providing a mud chamber fixedly connected to a mud supply by means of two flexible hoses, the supply of mud can be continued while tube parts are added to a tube or are separated therefrom.
Particularly advantageous elaborations of the invention are set forth in the dependent claims. Further objects, elaborations, effects and details of the invention appear from the following description, in which reference is made to the drawing.